The present invention relates to a plasma reactor. Silicon nitride films are commonly deposited using radial flow, planar plasma reactors. In a typical reactor configuration, the active and diluent gases flow radially inward from the periphery to the center of the reactor. As these gases pass through the plasma discharge, a silicon nitride film is deposited on the bottom of the powered electrode and the top of the grounded plate which holds the substrates.
In order to improve physical properties of the nitride film, such as intrinsic stress and density, it is desirable to increase the rf power input to the discharge. This increase in the rf power (e.g. to 400 W) has previously required the development of a gas inlet system which prevents premature decomposition of the reactant gases. Even with this improved inlet system, however, the nitride deposition rate can be much higher near the periphery of the substrate plate than toward its center.
For most process conditions it has been found that the nitride deposition rate and film properties vary with radial position on the substrate plate. This is presumably due to depletion of the reagent gas concentration as the reagent gases pass through the plasma. For low rf power levels (e.g. less than 100 watts) it is often possible to adjust the gas flows and reactor pressure to achieve the nearly uniform deposition rate over the entire area of the substrate holder. However, radial nonuniformity becomes a greater problem as higher power levels are used.
Higher power levels are desirable not only for higher throughput, but also for better material characteristics in some processes, such as deposition of nitride films. U.S. patent application Ser. No. 344,590, filed Feb. 1, 1982, abandoned in favor of continuation application Ser. No. 549,217, filed Nov. 7, 1983, discloses a plasma reactor improvement which permits higher power levels to be used without causing excessive undesired deposition of material in unwanted areas of the reactor. However, the increase in rf power levels permitted by the above-cited application causes the material deposition rate to be somewhat higher near the periphery of the substrate holder than near its center. In particular, the pressure and flow characteristics which are found to give the most desirable material characteristics do not give good uniformity.
Thus, it is an object of the present invention to provide a method for plasma-assisted chemical vapor deposition at high rf power which provides good uniformity of deposited material over the entire substrate holder area.